Such display devices are known and are usually operated by way of multiplex-addressing according to the so-called RMS mode.
The method of addressing (based on the so-called RMS behaviour of the liquid-crystal material) has been described, inter alia, by Alt and Pleshko in I.E.E.E. Trans. El. Dev. DE 21, 1974, pp. 146-155, by Nehring and Kmetz in I.E.E.E. Trans. El. Dev. ED 26, 1979, pp. 795-802, and by Kawakami et al. in SID-IEEE Record of Biennial Display Conference, 1976, pp. 50-52. This method of addressing is considered to be the most common for addressing liquid-crystal display devices which are constructed as a matrix of pixels as described above, in which use is not made of one active electronic switch (such as, for example, a thin-film transistor) per pixel.
Using this method of addressing, the pixels are switched, from a first state to an optically different second state with the aid of the line-scanning circuit which periodically scans the line electrodes using a line-select pulse of magnitude V.sub.s and with the aid of the control circuit for supplying data signals to the column electrodes, which control circuit applies data voltages of magnitude .+-.V.sub.d to the column electrodes over the time during which a line electrode is being scanned, in such a way that the optical state which is effected in a display element is determined by the so-called Root Mean Square (RMS) voltage value over the element in question.
The RMS voltage value V.sub.2 for the display elements not selected, i.e. the display elements in the ON state, is given by: EQU V.sub.2.sup.2 =(V.sub.s +V.sub.d).sup.2 /N+(N-1)*V.sub.d.sup.2 /N(1)
The RMS voltage value V.sub.1 for the display elements not selected, i.e. the display elements in the OFF state, is given by: EQU V.sub.1.sup.2 =(V.sub.s -V.sub.d).sup.2 /N+(N-1)*V.sub.d.sup.2 /N(2)
FIG. 2 shows, in diagrammatic form, a transmission voltage characteristic of a picture cell belonging to that display device.
Alt & Pleshko have derived relationships which, for a given value of the ratio S=V.sub.2 /V.sub.1 (sometimes called threshold slope in the transmission voltage characteristic curve) indicate the maximum number of lines N.sub.max which can be addressed by this method while maintaining a predefined contrast value and in what manner the voltage V.sub.s of the line-select pulse and the data voltages .+-.V.sub.d must be chosen to achieve this. These relationships are as follows: EQU N.sub.max ={(S.sup.2 +1)/(S.sup.2 -1)} (3) EQU (V.sub.s /V.sub.d).sup.2 =N.sub.max ( 4) EQU V.sub.d.sup.2 =V.sub.1.sup.2 *{0.5/(1-Q)} (5)
where: Q.sup.2 =N.sub.max
In the line-select voltage V.sub.s and the data voltage V.sub.d are then chosen in accordance with the expressions (2) and (3), the resulting RMS voltage, when using N.sub.max lines, over a selected pixel will be equal to V.sub.2, and the resulting RMS voltage over a non-selected pixel will be equal to V.sub.1.
A greater multiplexing rate, in other words a higher value for N.sub.max, requires a steeper slope of the transmission voltage characteristic curve, i.e. a value for the quantity S=V.sub.2 /V.sub.1 closer to 1.0.
By means of the currently known (and already used) so-called "SUPER-TWISTED" liquid-crystal effects it is possible to achieve very high values for N.sub.max, because the threshold slope S of the transmission voltage characteristic curve of these effects has a value which is very close to the limit value 1.0. FIG. 1, in diagrammatic form, shows part of a matrix-oriented display device 1 having N.sub.max selection lines (line electrodes) 2, and describes in principle the functioning of the RMS multiplex-addressing method mentioned earlier.
The information to be displayed is supplied to the data lines (column electrodes) 3. At the location of the crossing points of the selection lines 2 and the data lines 3 there are the display elements 4. Depending on the information supplied on the data lines 3, the display elements 4 are in an ON state or OFF state.
Synchronously with the selection of the lines or row electrodes with the aid of the line-select voltage V.sub.s (which has been selected in accordance with the expressions (4) and (5)), the image information (data voltage .+-.V.sub.d) is applied via the column electrodes. Thus, from the time t.sub.1 and over a period t.sub.1 (sometimes called line time), line 2.sup.a is selected which, together with the information then present on the data lines 3.sup.a, 3.sup.b, 3.sup.c (i.e. .+-.V.sub.d) determines the optical state of the pixels 4.sup.aa, 4.sup.bb, 4.sup.cc.
During this period t.sub.1 when line 2.sup.a is selected, all the other pixels corresponding to the line electrodes 2.sup.b, 2.sup.c etc. are at a voltage .+-.V.sub.d.
From time t.sub.2 (where: t.sub.2 -t.sub.1 =t.sub.1) line 2.sup.b is selected over period t.sub.1. The information then present on the data lines 3 (i.e. .+-.V.sub.d) determines the state of the pixels 4.sup.ba, 4.sup.bb, 4.sup.bc.
After said line time t.sub.1, the next line is then selected. Thus the whole picture is written line-by-line. After the last line of the matrix has been selected, the whole cycle is repeated (so-called "repeated scan procedure"). The duration of a single write cycle is called the frame time t.sub.f : t.sub.f =N * t.sub.1, N representing the number of lines which are thus scanned successively.
An important point with this RMS addressing method is that both the rise time and the fall time (in other words, the switching time for the transition to the `ON` or `OFF` state, respectively) of the optical effect are much greater than the frame time.
Under these circumstances, the display element reacts to the cumulative effect of a number of addressing pulses (or select pulses). Under these conditions, a liquid-crystal display element in particular reacts in the same way as if it had been addressed by a sinusoidal-wave signal or a stepped-wave signal or the like, having the same RMS voltage value as that of the `ON` and `OFF` voltages V.sub.2 and V.sub.1 given by the expressions (1) and (2).
As already discussed, the maximum number of selection lines N.sub.max is related to the value of the ratio V.sub.2 /V.sub.1 (threshold slope).
As the multiplexing rate increases, increasingly higher voltages are necessary when using the RMS multiplex-addressing method described above.
The line-select voltage V.sub.s in particular will become high (the data or column voltage V.sub.d in this addressing scheme should always be chosen to be lower than the threshold voltage of the optical effect).
The high select voltages result in the liquid-crystal effect no longer reacting to the RMS voltage value (RMS voltage mean over a frame time), but in the pixel showing an optical response which is determined by the `instantaneous` voltage value sensed by the element in question during the line-selection time. FIG. 3, in diagrammatic form, shows the transmission behaviour over one frame time of such a display element in the `ON` state.
The `ON` state is already reached during the line time t.sub.1 during which the element in question senses a voltage of magnitude V.sub.s +V.sub.d, because the `ON` switching time at sufficiently high voltages will become smaller than or equal to said line time. After selection, that is after the line time t.sub.1, the pixel in question only senses a voltage .+-.V.sub.d which is smaller than the threshold voltage. This means that the pixel in question, during the remaining frame time, will return to its `OFF` state. In FIG. 3 it is assumed, for the sake of simplicity, that said fall time (in other words, `OFF` switching time) is of the same magnitude as the frame time.
This characteristic so-called "non-RMS" transmission behaviour has been observed, inter alia, with liquid-crystal display devices having super-twisted liquid-crystal configurations, and particularly in the case of display devices having very thin liquid-crystal layer thicknesses. It is known that the switching times of a liquid-crystal effect depend strongly on said layer thickness. As the layer thickness becomes smaller, the switching time will decrease. See, inter alia, Okada et al. in SID Digest of Technical Papers XXII, 1991, pp. 430-433.
The outlined transmission behaviour (often referred to as "FRAME RESPONSE") in FIG. 3 leads to loss of brightness and contrast compared to a liquid-crystal effect reacting in a true RMS manner.